This invention relates to a method of controlling an air heater of a coal-fired boiler and, more particularly, to a method of controlling an air heater of a pulverized-coal fired boiler, which is intended for increasing the plant efficiency thereof.
Coal-fired boilers have been proposed and have been put into practical use for a long time. However, since many of these boilers were of a stocker combustion system requiring the disposal of combustion ashes, were difficult to control, use of such boilers was abandoned and instead oil-fired boilers became popular. However, due to the recent world-wide oil crisis coal-fired boilers have recently gained public attention. Thus, many of the boilers have been presently planned, designed, and constructed as pulverized-coal fired boilers.
In a pulverized-coal fired boiler, small coal pieces, prepared by pulverizing coal by a mill, are supplied into a furnace and are combusted therein; therefore it is necessary that no combustion ash is produced and control of the furnace is facilitated. The pulverized-coal fired boiler differs from other coal-fired boilers or oil-fired boilers in that it requires the use of a conveyance air for conveying the pulverized coal pieces from the mill to itself, in addition to the use of a combustion air. The at least two air supply systems are required with, one air supplying system supplying the conveyance air, generally called "primary air supplying system", and the other air supplying system supplying the combustion air, generally called "secondary air supplying system". A hot air, prepared by heating the atmospheric air, is used as the air to be supplied to boiler. If a cold air is supplied to the boiler as the combustion air, the boiler interior is cooled so that it is not possible to obtain a highly efficient combustion. Further, in order to convey the pulverized coal pieces by the primary air with high efficiency for combustion thereof, it is necessary that the content of water in the pulverized coal pieces is zero. The hot air is prepared with the use of an air heater which is so arranged as to effect a heat exchange between the atmospheric air and an exhaust gas of the boiler.
In, for example, U.S. Pat. No. 3,246,635, a pulverized-coal fired boiler is prepared having an air supplying system and, as shown in its FIG. 5, the cold air in the first stage, and the hot air in the second stage, of an air heater 9 are supplied into a mill 25 through dampers 26, 34, respectively. Air temperature in the mill 25 is monitored by a temperature sensor 60 so as to prevent an explosion in the mill due to too high air temperature and is adjusted to a predetermined temperature by controlling the openings of the cold and hot air dampers 26 and 34.
According to this known example, the air temperature in the mill is controlled to a predetermined value by controlling the mixing ratio between the primary hot air and the primary cold air. However, the water content in the coal varies widely in dependence upon the type of the coal, and when a coal which is high in water content is used, the air temperature in the mill is decreased, so that the operation is performed under a condition wherein a primary hot air damper is fully opened and a primary cold air damper fully closed. In this case, in order to quickly compensate for a coal with a high water content, it is necessary for the air heater to be sufficiently large so as to enable the primary hot air temperature to be increased. On the other hand, when a low water content coal is used, the operation is performed under conditions wherein the primary cold air damper is almost fully opened and the primary hot air damper almost fully closed. In this case, the air is again cooled, after being heated by the air heater, to a temperature which corresponds to a suitable within-mill air temperature. Therefore, the rate of heat exchange in the air heater is very low and is uneconomical.
An air underlying present invention essentially resides in providing a method of controlling an air heater of a coal-burning type boiler by which it is possible to obtain a necessary temperature of the primary air without employing a large capacity air heater while also effecting a high heat exchange.
In accordance with the control method the invention the flow rate of a cold air is restricted by maintaining a cold air damper, through which the cold air is taken in, almost fully closed, and under this condition to control a gas damper of the air heater so as to obtain a necessary temperature of the primary air after mixtured.
Thus, in accordance with the present invention, the flow rate of the cold air which is taken in is minimized and the flow rate of an exhaust gas into the air heater is increased. Therefore, it is possible to obtain the necessary temperature of the primary air without providing a large capacity air heater. Besides, the rate of heat exchange in the air heater can also be increased, whereby the heat of the exhaust gas can be effectively utilized.